Control system



. 17, 1946. J. G. AIVY CONTROL SYSTEM Filed June 14, 1944 2 Sheets-Sheet1 l .ma mw, mi.;

a* R zu 3 x @n z o INVENTOR WITNESSES:

2 Sheets-Sheet 2 8 Hg. e

INVENTOR Jae 614g'. BY

Mmu'w ATTORNEY Patented Dee. 17, 1946 CONTROL SYSTEM Joe G. Ivy, ForestHills, Pa., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Application June 14,1944, Serial No. 540,267

8 Claims. (Cl. 172-239) My invention relates to control system of thevariable voltage type and particularly to systems containing anauxiliary regulating generator for limiting the load current of a motorto be controlled, systems and generators of this kind being disclosed inmy copending applications Serial No. 496,596, filed July 29,1943, andSerial No. 532,975, filed April 27, 1944, both assigned to the assigneeof the present application.

An object oi my invention is to provide a control system of the typejust referred to that produces not only a load limiting regulatingeffect, but is al-so distinguished by increased accuracy and speed asregards its response vto changes in the control adjustment selected bythe attendant, While being free of appreciable hunting even at a highrate oi change of the control adjustments.

Another object of my invention, with reference to hoists, particularlymine hoists, is to provide a hoist control system in which the torque ofthe hoist motor and hence the stress on the ropes and other mechanicalhoisting equipment are automatically limited due to controlcharacteristics inherent in the electrlcalcontrol system, and in whichthe hoisting speed is va function of the selected controller positionregardless of the amount of load, thus reducing the degree of attentionand skill for operating the hoist, for instance, When landing a cage orpulling a skip into the dumping horns.

Still another aim of my invention is to achieve the above-mentionedobjects with the aid of auxi iliary generators which have a minimumnumber of field windings and low iield heating, and it is also intendedto reduce undesirable transients apt other field exciting meansoperating as described hereafter. (When referring to a self-excitedwinding in this specification, I use this term for designating a fieldwinding which is shunt or series connected to the appertaining generatorarmature and hence, in contrast to a "separately j excited winding,derives its excitation from the electric energy generated in thearmature of the same machine.) 'I'he other auxiliary generator,

hereinafter called the regulating generator has a saturationcharacteristic so that it generates `an appreciable armature voltageonly when its field excitation exceeds a given finite magnitude. Theregulating generator has a, field winding excited in dependence upon theload current of the motor to be controlled and is provided with otherfield exciting means also referred to hereinafter. I further equip thesystem with operator-controlled circuit means which provide anadjustable pattern voltage and also with automaticallyA operatingcircuit means which provide a control voltage variable in accordancewith the speed or Voltage of the motor to be controlled. These diiierentcircuit means are connected with each other and with the above-mentionedother neld exciting means of both auxiliary generators so as to producein each of them a component field excitation in dependence upon thedifferential Value of the pattern and control voltages.

According to another feature of my invention, the above-mentioneddifferential value of pattern voltage and control voltage is formedelectrically by superimposing both on a single electric circuit andimpressing the resultant differential voltage on a single iield windingof the two auxiliary generators, respectively.

These and other objects of my invention will be apparent from thefollowing description of the embodiment represented in the drawings, inwhich:

Figure 1 is the circuit diagram of a mine hoist design and operative inaccordance with my invention;

Figs. 2 and 3 are explanatory diagrams relating to the same system;

Fig. 4 is a partial view of a regulating generator used in the system ofFig. 1, the gure representing one of the pole structures of thisgenerator;

Fig. 5 is a partial circuit diagram of a second control system accordingto the invention; and

Fig, 6 is the circuit diagram of a third control -system also relatingto a hoist drive.

Referring to the hoist control system shown in Fig. 1, the hoistingequipment proper is schematically indicated at l and includes a hoistingdrum whose shaft 2 is mechanically connected to the armature I0 of adriving motor HM. The motor has a separately excited field Winding Iland a series-connected compensating winding I2. Winding Il is connectedthrough a rheostat RI with a suitable direct current source ofsubstantially constant voltage, here represented by an exciter generatorEX having an armature 50, two

aaiases iield windings l and 52, and an adjusting rheostat R7.

The amature i@ of the hoist motor HM is connected in load circuit Aiwith the armature 2@ of a main generator MG which forms .the currentsource of circuit Ai. The main generator MG has a main field winding 2iconnected in a control circuit A2 whose current source is formed by thearmature 33 of a control generator CG. A resistor R2 is series-arrangedin circit A2. Numeral 22 denotes a compensating or interpole windingofthe main generator MG.

The control generator CG has three iield windings 3|, 32, and 33 whichcooperate in controlling the voltage impressed on the main generator eldwinding 2|. The resultant energization of eld winding 2| determines thevoltage and current supplied by` .the main generator armature 2@ throughcircuit Al to the hoist motor HM, .thereby controlling the torque andspeed of the hoisting equipment driven by the motor HM.

operator. This voltagehereinaft er referred .to as the pattern voltage,is derived from the ex- .citer mains Iii and Y through reversing switchmeans RS and a potentiometric control device C0 whose resistor isdenoted by R3.

The reversing `switch RS, as illustrated, has

'two movable contacts il and ifi, each cooperating with two stationarycontacts i2, i3 and it, i3

respectively. In one position of switch RS, contact il is in engagementwith contact l2, andl trollr, while vthe reversing switch RS and its Thechange of current in circuit A2 is determined by the excitation of fieldwinding 3|, here-l inafter called the pattern field winding of thecontrol generator CG. Field Winding 32 is selfenergized and serves toamplify the control eilect. While the connection of winding 32 incircuit A2 is shown as a series arrangement, a shunt winding may be usedinstead. Winding 33 remains substantially deenergized as long as theload current in circuit AI remains below a given limit, but

when energized by the action of a regulating generator RG to bedescribed hereinafter, acts in opposition to eld winding 3|. That is,when winding 33 is excited, its ampere turns diminish the resultantfield effective on the armature 30 of control generator CG. Thisfunction .takes place when the load is positive, that is when HM acts asa drive and MG as a generator.. The-arrows shown in Fig. 1 adjacentto-windings 3| and 33, respectively, to indicate the directionalrelationship of the fields-induced by these windings refer to thejust-mentioned operating conditions. Under overhauling load conditions',HM will act as a generator and MG as a motor. The system will then stillbe operative to limit current, but the eld of winding 33 isthenoperative in the same sense as winding 3| and hence assist 4ratherthan buck the iield of the latter.

'I'he regulating generator RG has its amature 40 connected to'eldwinding 33 of control generator CG through a. rheostat Rwhoseadjustment-need not be changed during the operation EX are driven atsubstantially constant speed,

for instance, as shown in Fig. l, by means of a constant speed motor MIdriving a common shaft 3 of armatures 20 and 30, and another constantspeed motor M2 driving a shaft 4 common to armatures 40 and 50.

The above-mentioned control iield windings 3| and 4| of the auxiliarygenerators CG and RG, respectively, are both connected to a controlcircuit which isenergized from two independently variable voltagesources. One of these sources imposes on the circuit a constant voltagewhose magnitude and direction are adjustable by the contacts areYpreferably formed by electromagneticfcontactors which are controlled inaccordance with the selected hoist or lower position lof the mastercontroller. Since the particular design of the operator-actuated controlelements, here represented by CO and RS, is not essential for theinvention proper, and as such, need not involve novelty over the knownmaster control- 1ers, the simplied illustration in Fig. 1 has beenchosen for the sake of convenience and clarity.

The other voltage source of the control circuit Y of .windings 3| and 4|serves to impose on this circuit acomponent voltage, hereinafter calledthe control voltage, that varies automatically in accordance with thespeed of the hoist motor HM. In the embodiment of Fig. 1, the source of..this voltage consists of a pilot generator PG whose iield winding 6Iobtains constant excita- Vtion lfrom exciter mains X and Y through anadjusting rheostat R8, while its amature 60 is driven by,the motor shaft2 and hence generates a voltage proportional to the armature speed.

The pattern voltage and control voltage counteract each other so thatthe resultant voltage impressed on the control field windings 3| and 4I,

or the Voltage drop across either, corresponds tothe differential valueof these two component voltages. Consequently, when the speed o-f thehoist motor HM corresponds exactly tc the setting of`A lthe mastercontroller, the electrically established voltage differential is zero sothat both field windings 3| and 4I are deenergized. When the motor speeddeparts from the value set by the motor controller, a. positive ornegative differential volt- -age appears and causes the iield winding 3|of control generator CG to change the field excitation of the maingenerator MG in the direction required for correcting the discrepancy.This speed regulating eiect is extremely sensitive due to the action ofthe self-energized iield winding 32 which feeds a small portion of thecontrol generator output power back into the control generator asexcitation to produce still more output. The self-excited field is justsuiiicient to sustain the generated voltage. Hence,'the action of thecontrol generator is always under the control of one or more of theseparately excited eld vwindings 3| and 33. Stating this another way,the resistance of the external control circuit A2 is ad- Justed relativeto the self-excited iield excitation so that the machine CG operatesnear, or preferably on, its air gap line.

As stated above, the field winding 33 is deenergized under normalloadconditions but modifies the voltage of circuit A2, thus producing alimiting effect on vthe load current in circuit Al, when this currenttends to exceed a safe limit. In order to accomplish this action, theregulating generator RG is given a special characteristic similar tothat shown in Fig. 2. This characteristic differs Y from that ofconventional machines by having an extended low voltage or zero voltageinterval between points E and F which are both displaced from the zeropoint 0 by fixed amounts of excitation.

A characteristic ofthis type can be obtained by providing the fieldpoles of the generator with a saturable magnetic shunt as exempliiled byFig. 4.

In Fig. 4, numerals l, Il, and 80 denote the shaft, armature, and statorframe, respectively, of the regulating generator. The pole structurecomprises a base portion 8| in good magnetic contact with the statorframe, and a pole shoe portion 82 adjacent to the armature. The mainpole portion 83 is shunted by a saturable ilux path 85 and contains anair gap, or a shim 84 of high magnetic reluctance, for instance ofbrass. The windings of the generator are arranged on the main poleportion 83.

'Ihe air gap or shim at the top of the main pole position carries boththe armature exciting ux and the shunt flux, and hence does notiniluence the division of ux. It primarily determines the 1 spacing F-Eof the zero voltage interval as well ilows through the shunt, since thereluctance of' this path is low, and almost no flux is forced across theair gap into the armature to generate voltage, as long as the resultantileld excitation is insuicient to saturate the shunt path. The shunt 85is so proportioned that it saturates at an excitation corresponding topoints F and E of Fig. 2 and then becomes unable to carry appreciablymore ilux. Consequently, upon saturation of the shunt, an increasingfield excitation forces ilux across the air gap into the armature sothat now a voltage is generated. The action, after the shunt has beensaturated, is similar to that of a conventional mach/ine. Y

Due to the fact that ampere turns from the regulating field winding 45are used for saturatng the magnetic shunt, the generator output voltageis controlled in response to the armature current (and hence torque) ofthe hoist motor HM. As a result, the above-mentioned action between thecontrol elds and the current limiting field of the control generator CGis not effective before the magnetic shunt on the regulating generatorRG is saturated, and this saturation occurs only when the load currentof the hoist motor has exceeded a safe limit value. The control eldwinding li does not saturate the shunt because the pattern voltageequals, and thus balances out, the control voltage during normalsteady-state operation of the hoist motor. The transient departure fromzero of this control eld occurring during periods of changing speed isalone insuicient to saturate the shunt although these transient fieldeffects are operative to produce the regulatory anti-hunting motor HMapproaches full speed and the voltage of the main generator MGapproaches maximum,

the correcting effect required of the current limit winding 33 ofgenerator CG, in order to properly limit the current, will be lessbecause the control voltage of the pilot generator PG Willcancel agreater portion of pattern voltage and does not have to buck down asmany ampere turns from winding 3i as was previously -necessary due tothe larger difference between pattern voltage and control voltage. Theiield winding 4I of generator RG is now eil'ective to produce thisdesired adjustment in excitation of the current limit winding 33. Thatis, since the just-'mentioned differential value of control and patternvoltages is also effec- 'tive in field winding Il, the resultant fieldexcitation of generator RG and hence the current limiing effect ofwinding 33 are automatically reduced in the right amount to maintain thesame armature current in hoist motor HM. It is this simultaneousoccurrence, and its corrective eect. of the regulating action of thespeed-dependent and operator-adjusted voltages in both generators CG andRG that decreases the tendency of hunting, and thus increases theaccuracy, reliability` and permissible speed of control.

The control is completely reversible by actuating the switch RS and,when reversed, will limit regeneration in the same manner as describedabove with respect to current limitation.

Fig. 3 shows a theoretical speed torque characteristic I of a motorcontrolled by such a system. Curve T represents a test curve asobtainable, for instance, by point-to-point static loading. Anytransient overshooting of the current limit can be kept in the order of10% or less even on the severe test of plugging.

The advantages of this system of control include:

l. Better performance due to a closer approach to the ideal speed torquecharacteristic; 2. Less maintenance, due to the elimination ofcontactors and excessive commutating curl rents in the motors andgenerators; 3. Increased life oi the mechanical hoist parts,

due to less abuse from high torque (or current) peaks;

' 4. Great reliability, since the control system is As explained in theforegoing, it is essential for the invention that the eld excitations ofthe control generator CG and of the regulating generator RG arecontrolled in dependence upon the diieryentlal value of two componentvoltages, one (pattern voltage) being adjusted at will by the operatorwith the aid of the master controller, while the other component voltage(control voltage). varies substantially in accordance with the speed ofthe motor to be controlled. While in the embodiment of Fig. l the speedindicating voltage is produced by a pilot generator PG, there are othermeans available for producing a component voltage representative of themotor speed. The modification shown in Fig. 5 serves to exemplify one oithese other possibilities.

The system represented by Fig. 5 is partially identicalwith that of Fig.1 and hence illustratedV only to the extent necessary to indicate itsdifferences. Th'e main difference lies in the fact that the speedresponsive control voltage is not produced by a pilot generator but isderived by povtentiom'etric means from the load circuit of the hoistmotor HM. To this end. a potentiometer rheostat R9 is connected acrossthe armature it) of the hoist motor. The adjustment of this rheostatneed not be changed during the operation of the'hoist. The armaturevoltage is substa'ntially proportional to the motor speed. Hence, thevoltage drop between points t3 and ed in Fig. 5 is substantially ameasure of the motor speed. The terminal points t@ and ed in Fig.,5correspond to the similarly indicated points in Fig. 1, and the circuitof the master controller and of eld windings .3l and il isotherwiseexactly as shown in Fig. 1, with the exception that anv additional eldwinding 3d is placed on the control generatorCG and connected, through aresistor Rd', across the main generator winding 22 in order to correctthe resultant field excitation of windings 3i and Sli so that itcorresponds more accurately to the speed of motor HM. The operation ofthe system represented by Fig. 5 is otherwise substantially the same asdescribed above in'connection with Fig. 1.

Referring once more to Fig. i, it will be through a calibrating rheostatR8 with the master controller/ and the pilot' generator so that it isexcited in accordance with the differential valuev of pattern voltageand control voltage. The pattern field winding 3l of control generatorCG is also connected with the master controller and pilot generator soas to obtain likewisea com` ponent excitation in accordance with thejust= mentioned differential voltage value. The armature d@ of theregulating generator RG is con, nected between pilot generator- PG andpattern field winding 3l. Under normal operating conditions, the outputvoltage of the regulator armature il@ is negligible. Under overloadconditions, however, this output voltage reduces theresultantdifferential voltage across .winding 3l., and hence reduces theexcitation of the control generator CG, thus imposing thereon a currentlimiting efremembered that the output voltage of the regulatinggenerator RG, under operating conditions where a current limitation inthe load circuit of the motor HM is desired, causes the current limitingfield winding 33 of the control generator CG to buck by its ampere turnsthose of the field winding 3i. n other words, the regulating efect ofgenerator RG in the system of Fig. 1 functions by imposing a subtractivemagnetic eect on thel field excitation of the control regulator CG.According to another feature of my invention, however, it is alsopossible to have the, E. M: F.s produced by the differential effect ofthe pattern voltage and control voltage 'and by the output voltageof theregulating generator, respectively, act directly upon each other so thata single field winding in generator CG may take the place of bothwindings 3l and 33. A modication of this type is represented in Fig. 6.y

The hoist control system according to Fig. 6 is partially identical withthat of Fig. 1, as is apparent from the use of the same referencecharacters in both gures for corresponding elements. The hoist motor HMis energized by a main generator MG and has a separately excited fieldwinding I I connected by an adjusting rheostat Rl to the mains X and Yof a suitable direct-current source of substantially constant voltage.The separately excited field winding ll of the main generator MG isenergized by a control generator CG which, in contrast to the system ofFig. 1. has only two field windings, one being a self-excited orfeedback winding, denoted by 32,' while the,

other winding, denoted by 3|, is energized by a resultant voltage iscomposed as follows:

An operator-actuated master controller CO, including a reversing switchRS, serves to provide a selective pattern voltage. A pilot generator PGmounted on the shaft 2 of hoist motor HM, as in the system of Fig. l,provides a speed responsive control voltage. A regulating generator RGhaving its regulating ileld winding 45 energized through a rheostat R5in accordance with the load current of the hoist motor, also asdescribed more in detail in connection with Fig. 1, produces an armatureoutput voltage only when the load current exceeds a given maximum. Thewinding di of the regulating generator RG is connected fect similar tothe one obtained magnetically in the system of Fig. 1. Winding il ofgenerator RG, in the circuit connectionaccording to Fig. 6, is in effecta shunt connected self-energized winding. It is preferably so ratedrelative to the resistance of the armature circuit of this generatorthat the generator operates on its air gap line.

As exemplified in the foregoing, it is possible to lmodify controlsystems according to my invention as regards several of its detailswithout vdeparting from its spirit and its essential features as setforth in the appended claims.

Iclaim as my invention:

1. A variable voltage drive comprising, a motor, a main generator havingan armature circuit connected to said motor and a main generator fieldwinding for controlling the voltage of said circuit, an amplifyingcontrol generator for ener gizing said main generator eld winding havinga current limiting eld winding and a control eld winding,operator-controlled circuit vmeans for providing an adjustable patternvoltage, speedresponsive circuit means for providing a control voltagevariable substantially in accordance with the speed of said motor, saidoperator-controlled circuit means and said speed-responsive circuitmeans 'being connected with eachother and with i said control fieldwinding so as to energize said latter windingin accordance with thedifferential value of said pattern and control voltages, a regulatinggenerator having saturably shunted field poles so as to generate anoutputvoltage only when the excitation of said eld poles exceeds a givenvalue, means connecting said regulating generator with said armaturecircuit for providing field excitation for said regulating generator independence upon the load current in said armature circuit, saidregulating generator being connected to said current limiting eldWinding so as to impress said output voltage on the latter in order tolinut said load current when said load current tends to exceed a safelimit value.

2. A variable voltage drive comprising, a motor, a main generator havingan armature circuit connected to said motor and a main generator fieldwinding for controlling the. voltage of said circuit, an amplifyingcontrol generator disposedl for energizing saidmain generator fieldwinding and having a current limiting eld winding and a control vf-leldwinding, a regulating generator having saturably shunted. field poles soas to generate voltage only when the excitation of said field polesexceeds a given Value, said regulating generator being held-controlledby said armature circuit and connected to said Acurrent limiting fieldwinding so as to impress on the latter said voltage when the loadcurrent in said armature circuit exceeds a desired maximum value, acontrol field winding on said regulating generator, operator-controlledcircuit means for providing an adjustable pattern voltage, circuit meansfor taneously in accordance with the differential value of said patternand control voltages.

3. A variable voltage drive comprising, a motor, a main generator havingan armature circuit connected to said motor and a main generator fieldwinding for controlling the voltage of said circuit, an amplifyingcontrol generator disposed for energizing said main generator eldwinding and having a current limiting eld winding and a control eldwinding, a regulating generator having saturably shunted field poles soas to generate voltage only when the excitation of said field polesexceeds a given value, said regulating generator being held-controlledby said armature circuit and connected to said current limiting eldwinding so as to impress on the latter said voltage when the loadcurrent in said circuit exceeds a given maximum limit, a control eldwinding on said regulating generator, operatorcontrolled circuit meansfor providing an adjustable pattern voltage, and a pilot generatormechanically connected with said motor for providing a control voltagevariable substantially iny accordance with the motor speed, said circuitmeans and said pilot generator being connected -with each other and withsaid two control eld windings so as toV energize them simultaneously inaccordance with the diierential value of said pattern and controlvoltages.

.4. A variable voltage drive comprising, a motor, a. main generatorhaving an armature circuit connected. to said motor and a main generatoreld winding for controlling the voltage of said circuit; an amplifyingcontrol generator disposed for energizing said main generator i'leldwinding and having a current limiting field winding. and a control eldwinding, a regulating generator having saturably shunted field poles soas to generate 'voltage only when the excitation of said leld polesexceeds a given value, said regulating generator being deliri-controlledby said armature circuit and connected 'to said current lini'ing fieldwinding so as to impress on the latter said voltage when the loadcurrent in said circuit exceeds a desired maximum value, a control deldWinding on said regulating generator, operatorcontrolled circuit meansfor providing an adjustable pattern voltage, and voltage measuringcircuit means connected with said armature circuit for providing acontrol voltage variable substantially in accordance with the motorspeed, said former and said latter circuit means being connected witheach other and with said two con-l trol eld windings so as to energizethem in accordance with the diierential value of said pattern andcontrol voltages.

5. A variable voltage drive comprising, a drive motor, a main generatorhaving an armature circuit connected to said motor and a main generatorfield winding for controlling the voltage of said circuit, a controlgenerator disposed for annees energizing said main generator fieldwinding and A winding and separately excited eld winding means, aregulating 'generator having saturably shunted eld poles for generatingabruptly increasing regulating voltage in dependence upon the passing ofits field excitation through a shuntsaturating value and being providedwith eld winding means and having a regulating iield winding connectedwith said circuit for provid ing excitation in accordance with the loadcurrent of said motor, operator-controlled circuit means for providingan adjustable pattern voltage, circuit means for providing a controlvoltage variable substantially in accordance with the speed of saidmotor, said former and said latter circuit means being connected witheach other and with said field winding means of said control generatoras well as with said field winding means of said regulating generator soas to energize said respective eld winding means simultaneously inaccordance with the differential value of said pattern and controlvoltages, and said eid winding means of said control generator beingalso connected with said regulating generator so that the resultant eldexcitation of said control generator is modied due to said regulatingvoltage when said load curren exceeds a desired maximum value.

6. A variable voltage drive comprising, a motor, a main generator havingan armature circuit con nected to said motor and a main generator fieldwinding for controlling the voltage of said circuit, a control generatordisposed for energizing said main generator eld winding and having aself-excited voltage-sustaining ileld winding and a current limiting eldWinding and a control field Winding, operator-controlled circuit meansfor providing an adjustable pattern voltage, circuit means for providinga control voltage variable substantially in accordance with the speed ofsaid motor, said former and said latter circuit means being connectedwith each other and with said control field winding so as to energize itin accordance with the differential value of said pattern and controlvoltages, a regulating generator having saturably shunted eld poles forgenerating abruptly increasing output voltage in dependence upon thepassing or its :field excitation through a shunt-saturating value andbeing connected with said current limiting elrl winding so as to impresssaid outn put voltage thereon ier reducing the resultant eld excitationof control generator when the load current of said motor exceeds adesired maximum value, and circuit means connected to said armaturecircuit for supplying :iield excita-v tion to said regulating generatorin dependence upon said load current.

7. A variable voltage drive comprising, a motor, a main generator havingan armature circuit connected to said motor and a main generator ieldwinding for controlling the voltage of said circuit, an amplifyingcontrol generator disposed for energizing said main generator iieldwinding and having a self-excited voltage-sustaining iield winding and acurrent limiting field winding and a control eld winding, a regulatinggenerator having saturabli7 shunted iield poles for generating abruptlyincreasing output voltage in dependence upon the passing of its eldexcitation through a shunt-saturating value and being connected withsaid current limiting ileld winding, said regulating generator having acontrol iield il winding and a regulating eld winding.operatorcontrolled circuit means for providing an adjustable patternvoltage, circuit means for providing a control voltage variablesubstantially in accordance with the speed of said motor, said formerand said latter circuit means being connected with each other andwithsaid two control field windings so as to energize them in accordancewith the diierential value of said pattern and control voltages, circuitmeans connected with said regulating winding for energizing it independence upon the load current of said motor so that said currentlimiting winding is energized only when said load current exceeds apredetermined maximum limit, and said current limiting iield windingbeing arranged to act in opposition to said control eld winding of saidcontrol generator so that said output voltage causes said currentlimiting ileld winding to reduce the resultant field excitation of saidcontrol generator, 8. A variablevoltage drive comprising. a drive ofsaid circuit, a control generator for energizing said main generator eldwinding having eld winding means, a regulating generator havingsaturably shunted eld poles for generating abruptly increasingregulating voltage in dependence upon the passing of its cld excitationthrough a shunt-saturating value and being provided with eld windingmeans and having a regulating eld winding connected with said circuitfor providing excitation in accordance with the load current of saidmotor, operator-con- -trolled circuit means for providing an adjustablepattern voltage and circuit means for providing a control voltagevariable substantially in accordance with the speed of said motor, saidformer and said latter circuit means being connected with each other andwith said iield winding means of said control generator as well as withsaid eld winding means of said regulating t generator so as to energizesaid respective iield generator is reduced due to said regulatingvoltage when said load current exceeds a given maximum value.

JOE G. IVY.

